If Roeder, 51, can persuade jurors that he had an "unreasonable but honest belief" that killing Tiller was necessary to save lives, Kansas law allows them to convict him of voluntary manslaughter rather than the more serious crime of murder.

But the judge ruled that he would wait to hear the evidence before deciding whether to give jurors the voluntary manslaughter option.

"We don't fast forward," Wilbert said. "We don't jump to conclusions, and we don't arrive at the end of the process without a full and complete -- and hopefully impartial -- hearing."

The judge's ultimate ruling is sure to stir the controversy that has long surrounded abortion in this country.

Abortion opponents took Tuesday's decision as a victory, but it would be a major triumph if the judge were to allow the jury to consider voluntary manslaughter, because he would be implying that fetuses are human beings whose lives can be saved, according to legal experts.

Kansas courts have ruled that viable fetuses are not human beings.

"If the judge allows this, he would be allowing the jury to consider whether a fetus is a person," University of Kansas law school professor Suzanne Valdez said.

Abortion-rights advocates also say giving juries the option of applying a lesser charge to the killers of abortion providers would encourage more violence against the providers.

Scott Roeder Argues Killing Abortion Doctor Saved Lives

Roeder has admitted in legal documents that he committed the killing, but he has consistently maintained that his actions saved countless unborn children and was justified.

In other cases involving the killing of abortion providers, the suspects have argued that their actions were necessary to save lives.

Many states allow the so-called necessity defense as a complete defense to murder charges when killing one person is necessary to save the life of someone else who is in imminent danger.

For example, Paul Hill, a former minister who killed abortion provider Dr. John Bayard Britton in 1994 in Pensacola, Fla., tried to raise the necessity defense during his murder trial, but the judge refused to allow it. Hill was convicted and, in 2003, executed.

Roeder is making a more nuanced argument. He is not saying that it was necessary to kill Tiller to save the lives of the unborn, but that he believed it was necessary, even though that belief may have been completely unreasonable.

So, he argues, he should not be convicted of murder but of voluntary manslaughter, defined in Kansas as the "unreasonable but honest belief that circumstances existed that justified deadly force" during an intentional killing. This is known as an imperfect self-defense.

At Tuesday's hearing, prosecutors asked the judge to prevent Roeder from making that argument because it required Roeder to believe that Tiller presented an imminent threat to fetuses, and clearly no such threat existed: Roeder gunned down the doctor while he was at church, not at his clinic.

The jury should only consider whether Roeder's actions were premeditated, the prosecutors said.

But defense lawyers countered that it did not matter how clear the absence of an imminent threat was, because imperfect self-defense by definition requires the killer's belief to be unreasonable. In any case, they argued, the threat was far more than just a fear of future harm to fetuses.

"There was a state-licensed facility operating in Sedgwick County (Kansas) to perform abortions," wrote defense attorney Mark Rudy in papers filed with the court. "It had staff. It had a practitioner (Tiller). It had a budget. ... In the mind of Mr. Roeder, the victim presented a clear danger to unborn children."